1. Field of the Invention
This invention relates to a method of forming coupling head portions on a monofilament of thermoplastic synthetic resin in the production of a slide fastener coupling element.
2. Description of the Related Art
A method for stamping a monofilament of synthetic resin for slide fastener coupling elements is known from, for example, Japanese Patent Publication 8902/1988. In the known method, as shown in FIGS. 13(a) and 13(b) of the accompanying drawings, a monofilament A of synthetic resin is continuously stamped on opposite sides by a pair of rotating rollers B, B to provide longitudinally spaced successive pairs of depressed small-thickness portions C, C, as the monofilament A is continuously supplied between the two rollers B, B. The stamped monofilament A is then continuously bent at opposite ends of the individual small-thickness portion C to form coupling head portions of a coiled filamentary coupling element.
Practically, however, the stamped monofilament A tends to be bent along the individual small-thickness portion on the side toward an imaginary center line passing centrally of its length, the gap between the upper and lower leg portions on the side toward the coupling head portion is apt to be reduced. In a slide fastener using the resulting coupling elements, smooth coupling and uncoupling of a pair of opposite rows of coupling head portions cannot be performed when a slider is moved on and along the slide fastener. Also it is difficult to move the slider smoothly. Further, after coupling, the slide fastener would not be kept in a stably coupled posture.